This invention relates to electronic circuitry and, more particularly, to a charge pump for producing a negative substrate bias in a complementary metal oxide semiconductor (CMOS) integrated circuit.
MOS transistors are commonly used in electronic circuits such as dynamic random access memories (DRAMS). In an NMOS transistor, an N-type source region is separated from an N-type drain region by a P-type channel region. All three regions are formed in a P-type semiconductor substrate. By applying a positive voltage to a gate electrode disposed above the channel region, electrons gather in the channel region between the source region and the drain region to allow current to flow from the drain region to the source region. PMOS transistors have the same structure except the conductivity types of the various regions are reversed and a negative gate voltage is required to allow current to flow from the source region to the drain region.
It has been found that NMOS transistors operate better when the P-type substrate of the NMOS (or of the NMOS transistors in a CMOS circuit) is driven negative with respect to circuit ground, in other words there is a negative substrate bias. Such a negative substrate bias provides a number of advantages in terms of the overall circuit performance. More specifically, a negative substrate bias decreases the NMOS transistor source and drain capacitance, decreases the likelihood of latchup, decreases PN diode injection when a node is driven below ground, and decreases the effective body effect, all of which are desirable in CMOS circuits.
Typically a charge pump circuit is used to create the negative substrate bias. Once a negative substrate bias is achieved, however, it does not last forever. For example, when an NMOS transistor is conductive with a relatively high drain to source voltage, some of the electrons traveling from the source region to the drain region collide with atoms in the channel region with enough energy to cause electron/hole pairs to form. The positive gate voltage attracts the generated electrons to the surface of the channel while the positive drain voltage attracts them to the drain where they simply add to the normal flow of electrons from source to drain. The positively charged holes, by contrast, are repelled by the positively charged gate away from the channel region into the substrate. The substrate current created by the excess holes makes the substrate more positively charged, thus counteracting the negative substrate bias. In DRAMS, a substantial amount of substrate current is generated whenever the memory is read or written, since many transistors are switched on and off at that time. This component of substrate current may be orders of magnitude above the background (i.e., standby) leakage current of all the reverse biased P-N diodes throughout the circuit. Therefore, the charge pump must remove low substrate current during standby and high substrate current during high activity to maintain the negative substrate bias.
FIG. 1 is a conceptual schematic diagram of a charge pump 2 which includes a first switch 4 coupled between a positive power supply voltage (V.sub.cc) and a first terminal 6 of a capacitance C1. A second switch 8 is coupled between a ground potential (V.sub.ss) and a second terminal 10 of capacitance C1. A third switch 12 is coupled between (V.sub.ss) and terminal 6 of capacitance C1, and a fourth switch 14 is coupled between the substrate (represented by the voltage (V.sub.bb)) and terminal 10 of capacitance C1. In operation, switches 4 and 8 are both closed (made conductive) for charging capacitance C1 to a voltage equal to the difference between (V.sub.cc) and (V.sub.ss). In FIG. 1, (V.sub.cc)=+5 volts and (V.sub.ss)=0 volts, so capacitance C1 charges with node 6 five volts more positive than node 10. Thereafter, switches 4 and 8 are opened and switches 12 and 14 are both closed. Since the positive terminal 6 of capacitance C1 is now coupled to a ground potential, the negative terminal 10 of capacitance C1 tries to drive V.sub.bb to negative 5 volts through switch 14. Thereafter, switches 12 and 14 are opened, and the sequence repeats itself. An oscillator (not shown) typically controls the repetitive switching sequence, and a detector (not shown) monitors the substrate voltage and controls the pumping operation to maintain the substrate at the proper negative voltage level.
As discussed in more detail below, known charge pumps consume a substantial amount of power (often 1 milliwatt or more even when no further pumping is required), often work against themselves by adding positive substrate current as they operate, and generally operate inefficiently.